This invention relates to continuous casting of steel strip in a strip caster, particularly a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which molten metal flows through a metal delivery nozzle located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures of the order of 1400° C. and can suffer very rapid scaling due to oxidation at such high temperatures. Such scaling may result in a significant loss of steel product. For example, 3% of a 1.55 mm thick strip (typical scale thickness 23 microns) can be lost from oxidation as the strip cools. Moreover, such scaling results in the need to descale the strip prior to further processing by pickling to avoid surface quality problems such as rolled-in scale, and causes significant extra complexity, cost and environmental concerns. For example, the hot strip material may be passed directly to a rolling mill in line with the strip caster and thence to a run out table on which it is cooled to coiling temperature before it is coiled. However, scaling of the hot strip material emerging from the strip caster progresses so rapidly that it may be necessary to install descaling equipment to descale the material immediately before it enters the in line rolling mill. Even in cases when the strip is cooled to coiling temperature without hot rolling, it will generally be necessary to descale the strip either before it is coiled or in a later processing step.
To deal with the problem of rapid scaling of strip emerging from a twin roll strip caster, it has been proposed to enclose the newly formed strip within a sealed enclosure, or a succession of such sealed enclosures, in which a controlled atmosphere or atmospheres is maintained in order to inhibit oxidation of the cast strip. The controlled atmosphere can be produced by charging the sealed enclosure or successive enclosures with non-oxidizing gases. Such gases can be inert gases such as nitrogen or argon or exhaust gases from fuel burners.
U.S. Pat. No. 5,762,126 discloses an alternative relatively cheap and energy efficient way of limiting exposure of the high temperature strip to oxygen. The strip is caused to pass through an enclosure where oxygen is extracted from the atmosphere by the formation of scale. The enclosure is substantially sealed so as to control the ingress of oxygen into the enclosure atmosphere and control the extent of scale formation. In this method of operation, it is possible to rapidly reach a steady state condition in which scale formation is brought to low levels without the need to deliver a non-oxidizing or reducing gas into the enclosure.
U.S. Pat. No. 5,816,311 discloses a way of controlling the extent of scale formation by providing downstream a chamber where groups of nozzles spray a quenching medium onto the strip. The quenching medium was a methyl alcohol, water, or mixture of methyl alcohol and another quenching medium liquid at room temperature. It was expected that water spraying in a nitrogen atmosphere would lead to unacceptable levels of oxidation as water contains dissolved oxygen and the breakdown of water (steam) to oxygen and hydrogen would provide further oxidation; however, it was surprisingly and unexpectedly found as described in the '311 patent that it was possible to limit the thickness of oxide on the strip to no more than 0.5 microns. Additionally, it was surprisingly found that these levels of oxide were tolerable for cold rolling without pickling and then metal coating of the strip. This quenching of the steel strip was found, however, to result in uneven cooling of the steel strip introducing stresses and other defects in the strip.
International Patent Application PCT/AU00/01478, on which parent application Ser. No. 10/121,567 is based, discloses how a substantially non-oxidizing atmosphere can be cheaply and effectively produced within a downstream enclosure, through which the hot cast steel strip passes, by introducing water in a fine mist spray to generate steam within the enclosure. The steam generation increases the gaseous volume within the enclosure so as to produce a positive pressure in the enclosure which substantially prevents the ingress of atmospheric air. It can also produce an increased level of hydrogen gas within the enclosure to significantly reduce the oxygen level in the enclosure and reduce the rate of oxidation of the strip. In the disclosure of International Application PCT/AU00/01478 it was considered necessary to isolate the enclosure in which steam is generated from the enclosure to which the casting rolls are exposed so as to avoid the risk of exposure of the casting pool to water or steam. We have now found, surprisingly, that by the introduction of water in a fine mist spray, the conversion of the water to steam and the production of hydrogen gas is so effective that it is possible to generate increased levels of hydrogen gas in an enclosure to which the casting rolls are exposed, either by allowing communication with gas flow between that enclosure and the downstream enclosure into which the fine mist spray is introduced and/or by direct introduction of a fine mist spray into the enclosure to which the casting rolls are exposed. By direct introduction of the fine mist spray into the enclosure to which the casting rolls are exposed, it is also possible to omit the separate downstream enclosure.